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prop/algo1.md
Graham McIntire c11e6f06f4
add first algo
2026-03-29 14:26:42 -05:00

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Comprehensive Propagation Algorithm Documentation

Overview

The ntms-proptools suite consists of three interconnected applications for mm-wave (24/47 GHz) propagation analysis and prediction. All share common physics algorithms calibrated against confirmed radio contacts.

Calibration Contacts:

  • 47 GHz Jun 01 2024 98.8 km (09:00 CDT)
  • 47 GHz Nov 01 2025 116.0 km (07:44 CST)
  • 24 GHz Nov 01 2025 101.0 km (07:54 CST)
  • 24 GHz Sep 07 2002 542.1 km (longest, 07:35 CDT)

PART 1: PROP-CAST (Real-Time Station Scoring)

Purpose

Live multi-station propagation scoring across ~500-mile DFW radius using current ASOS observations.

Data Inputs

Surface Observations (ASOS/NWS):

  • Temperature (°F → °C conversion: (tF - 32) * 5/9)
  • Dew point (°F → °C)
  • Relative humidity (%)
  • Wind speed (knots → m/s: knots * 1.15078)
  • Wind direction (degrees)
  • Sea-level pressure (hPa)
  • Altimeter setting (inches Hg → hPa: inches * 33.8639)
  • Cloud cover (categorical: CLR, FEW, SCT, BKN, OVC, VV)

Frequency Configurations:

// 47 GHz Configuration
47: {
  label: "47 GHz",
  accentHex: "#00ffa3",
  attenPerKm: (rho) => 0.006 * rho + 0.040,
  humPenalty: 1.0,
  seasonAdj: { 6:-0, 7:-0, 8:-0, 5:-0, 9:-0 },
  // Atmospheric window: between 22.235 GHz H₂O peak and 60 GHz O₂ peak
}

// 24 GHz Configuration
24: {
  label: "24 GHz",
  accentHex: "#44ddff",
  attenPerKm: (rho) => 0.0067 * rho + 0.006,
  humPenalty: 1.6,
  seasonAdj: { 6:-8, 7:-10, 8:-10, 5:-4, 9:-4 },
  // Adjacent to 22.235 GHz H₂O absorption line shoulder
}

Physics Calculations

1. Absolute Humidity Calculation

function absHumidity(rhPct, tempC) {
  // Magnus formula for saturation vapor pressure
  const es = 6.112 * Math.exp((17.67 * tempC) / (tempC + 243.5));
  // Absolute humidity in g/m³
  return (217 * (rhPct/100) * es) / (tempC + 273.15);
}

Formula: ρ = 217 * (RH/100) * e_s / T

  • RH: relative humidity (%)
  • e_s: saturation vapor pressure (hPa) via Magnus formula
  • T: absolute temperature (K)
  • Result: ρ in g/m³

2. Gaseous Absorption by Frequency

attenPerKm(rho) = O₂_coefficient + H₂O_coefficient * rho

47 GHz: 0.040 + 0.006·ρ dB/km

  • O₂ component: ~0.040 dB/km (from 60 GHz O₂ line wing)
  • H₂O component: ~0.006·ρ dB/km (22.235 GHz H₂O line wing)

24 GHz: 0.006 + 0.0067·ρ dB/km

  • O₂ component: ~0.006 dB/km
  • H₂O component: ~0.0067·ρ dB/km (stronger H₂O sensitivity on 22.235 GHz shoulder)

Scoring Functions

All return 0-100 scale representing propagation favorability.

Score: Humidity

function scoreHumidity(rho, freq) {
  const r = rho * FREQ_CONFIG[freq].humPenalty;
  if (r <= 6)   return 100;
  if (r <= 9)   return 95  - ((r-6)/3)  * 20;
  if (r <= 13)  return 75  - ((r-9)/4)  * 30;
  if (r <= 18)  return 45  - ((r-13)/5) * 35;
  return Math.max(0, 10 - (r-18)*2);
}

Penalty Factor per Frequency:

  • 47 GHz: multiplier = 1.0
  • 24 GHz: multiplier = 1.6 (more sensitive due to H₂O line proximity)

Thresholds:

  • r ≤ 6: Score 100 (dry/excellent)
  • r ≤ 9: Linear decay from 95 → 75 (excellent → good)
  • r ≤ 13: Linear decay from 75 → 45 (good → marginal)
  • r ≤ 18: Linear decay from 45 → 10 (marginal → poor)
  • r > 18: Score 0 (very humid/negligible)

Score: Wind Speed

function scoreWind(mph) {
  if (mph < 2)  return 100;
  if (mph < 5)  return 90;
  if (mph < 9)  return 72;
  if (mph < 14) return 42;
  if (mph < 20) return 16;
  return 0;
}

Rationale: Wind causes mechanical mixing of boundary layer, destroying temperature inversion. Still air promotes stable inversion.

Score: Sky Cover

function scoreSky(c) {
  return ({
    CLR: 100, SKC: 100,
    FEW: 88,
    SCT: 60,
    BKN: 25,
    OVC: 5, VV: 5
  })[c] ?? 50;
}

METAR Cloud Cover Legend:

  • CLR/SKC: Clear (100)
  • FEW (1/8–2/8): 88
  • SCT (3/8–4/8): 60 (scattering)
  • BKN (5/8–7/8): 25 (broken)
  • OVC (8/8): 5 (overcast)
  • VV (vertical visibility): 5

Score: Season (Month-Based)

function scoreSeason(month, freq) {
  const b = ({
    1:88, 2:84, 3:72, 4:62, 5:55,
    6:42, 7:28, 8:28, 9:52, 10:68,
    11:96, 12:88
  })[month] ?? 50;
  return Math.max(0, Math.min(100, b + FREQ_CONFIG[freq].seasonAdj[month]??0));
}

Base Monthly Scores:

  • Peak: November (96), December (88), January (88)
  • Summer worst: July/August (28)
  • 24 GHz additional penalties: June (-8), July (-10), August (-10), May (-4), September (-4)

Rationale: Seasonal humidity variation; summer monsoon moisture increases H₂O absorption.

Score: Time of Day (Inversion-Based)

function scoreTimeOfDay(utcH, utcM, month) {
  const isCDT = month >= 3 && month <= 10;
  const local = (utcH + utcM/60 + (isCDT ? -5 : -6) + 24) % 24;
  const sr = [7.4,7.3,7.0,6.7,6.35,6.25,6.35,6.65,6.9,7.1,7.35,7.45][month-1];
  const d = local - sr;  // hours relative to sunrise

  if (d >= -0.5 && d <= 1.5) return {s:100, lbl:"Peak — inversion maximum"};
  if (d > 1.5 && d <= 3.0)   return {s:78,  lbl:"Good — inversion eroding"};
  if (d > -1.5 && d < -0.5)  return {s:82,  lbl:"Pre-sunrise — building"};
  if (d > 3.0 && d <= 6.0)   return {s:38,  lbl:"Marginal — BL mixing"};
  if (local >= 21 || local <= 3) return {s:68,lbl:"Overnight — cooling"};
  if (d > 6.0) return {s:18,lbl:"Afternoon — duct unlikely"};
  return {s:48,lbl:"Early morning"};
}

Window Tiers:

  1. Peak Window (sunrise -30min to +1.5h): Score 100 — Inversion maximum strength
  2. Good Window (+1.5h to +3h after sunrise): Score 78 — Inversion still present but eroding
  3. Pre-Sunrise (-1.5h to -30 min): Score 82 — Inversion building under clear skies
  4. Marginal Window (+3h to +6h after sunrise): Score 38 — Convective boundary layer growth
  5. Overnight Cooling (21:00 to 03:00 local): Score 68 — Weak inversion reformation
  6. Afternoon (+6h after sunrise): Score 18 — Full convective mixing, inversion destroyed

Sunrise Lookup Table (Monthly, local decimal hours):

Jan:7.4  Feb:7.3  Mar:7.0  Apr:6.7  May:6.35  Jun:6.25
Jul:6.35 Aug:6.65 Sep:6.9  Oct:7.1  Nov:7.35  Dec:7.45

Score: Temperature-Dew Point Depression

function scoreTdDep(tF, tdF) {
  const d = tF - tdF;
  if (d > 22) return 96;
  if (d > 14) return 80;
  if (d > 8)  return 60;
  if (d > 4)  return 38;
  return 18;
}

Rationale: Large Td depression indicates dry air above surface, favorable for stable stratification.

Thresholds (°F):

  • 22°F: 96 (very dry aloft)

  • 14°F: 80 (good stability)

  • 8°F: 60 (moderate)

  • 4°F: 38 (marginal)

  • ≤4°F: 18 (humid aloft)

Score: Pressure Tendency

function scorePressure(mb, prev) {
  if (prev == null) return 58;
  const d = mb - prev;
  if (d > 2.5) return 98;  // Rising pressure → anticyclone → stable
  if (d > 0.8) return 82;
  if (d > -0.5) return 60;
  if (d > -2.0) return 32;
  return 10;              // Falling pressure → cyclone → unstable
}

Composite Propagation Score

function composite(f) {
  return Math.round(
    f.humidity  * 0.26 +
    f.wind      * 0.18 +
    f.sky       * 0.15 +
    f.timeOfDay * 0.14 +
    f.tdDep     * 0.11 +
    f.season    * 0.09 +
    f.pressure  * 0.07
  );
}

Weights:

Factor Weight Importance
Humidity 26% Most critical (absorption)
Wind 18% Destroys inversion
Sky 15% Less impact than humidity
Time of Day 14% Inversion strength varies
Td Depression 11% Aloft stability
Season 9% Long-term trend
Pressure 7% Weak indicator

Total: 100%

Scoring Tiers

Score Band Label Hex
80-100 EXCELLENT Green/cyan #00ffa3
65-79 GOOD Light cyan #7dffd4
50-64 MARGINAL Yellow #ffe566
33-49 POOR Orange #ff9044
0-32 NEGLIGIBLE Red #ff4f4f

PART 2: PROP-ANALYZER (Historical Contact Analysis)

Purpose

Analyze atmospheric conditions (ASOS + Skew-T sounding) during historical radio contacts to identify propagation mechanisms.

Additional Physics Calculations

Refractivity Profile (Modified N-Units)

// Saturation vapor pressure (Buck equation)
function satVapPres(tC) {
  return 6.1121 * Math.exp((18.678 - tC/234.5) * (tC/(257.14 + tC)));
}

// N-unit profile (ITU-R P.453-14)
const N = 77.6 * P/T + 3.73e5 * e / (T * T)

// M-unit profile (accounts for earth curvature)
const M = N + 0.157 * (hght - sfc_hght)

Where:

  • P: pressure (hPa)
  • T: absolute temperature (K)
  • e: water vapor pressure (hPa) = satVapPres(Td)
  • h: height MSL (m)
  • sfc_hght: surface height MSL (m)

Duct Detection via M-Profile

Definition: A duct exists where dM/dh < 0 (M decreases with height)

const ducts = [];
let inDuct = false, ductBase = null, ductBaseM = null;

for (let i = 1; i < refractProfile.length; i++) {
  const dM = refractProfile[i].M - refractProfile[i-1].M;

  if (dM < 0 && !inDuct) {
    inDuct = true;
    ductBase = refractProfile[i-1].hAGL;
    ductBaseM = refractProfile[i-1].M;
  } else if (dM >= 0 && inDuct) {
    inDuct = false;
    const top = refractProfile[i-1].hAGL;
    const strength = ductBaseM - refractProfile[i-1].M;
    if (strength > 2) {  // Filter: only significant ducts
      ducts.push({ base: ductBase, top, strength: strength.toFixed(1) });
    }
  }
}

M-Unit Gradient Interpretation:

  • dM/dh < -157 /km: Super-refractive (ducting probable)
  • dM/dh < -79 /km: Enhanced refraction (extended range)
  • dM/dh > 0: Sub-refractive (reduced range)
  • dM/dh ≈ -40 /km: Standard atmosphere

Inversion Detection

const inversions = [];

for (let i = 1; i < sorted.length; i++) {
  if (sorted[i].tmpc > sorted[i-1].tmpc) {  // Temperature increases with height
    inversions.push({
      base: sorted[i-1].hght - sfc.hght,
      top: sorted[i].hght - sfc.hght,
      strength: sorted[i].tmpc - sorted[i-1].tmpc,  // °C
      rate: strength / ((top - base) / 100)  // °C per 100m
    });
  }
}

// Merge adjacent inversions within 200m gap
// Keep only meaningful: strength >= 0.5°C and base < 5000m

Stability Indices

K-Index:

const kIndex = (T850 - T500) + Td850 - (T700 - Td700)

Lifted Index (Simplified):

const li = T500 - (Tsfc - (p500_hght - sfc_hght) * 0.00976)
  • LI < 0: Unstable (convection likely)
  • LI > 0: Stable (inversion maintained)

Precipitable Water:

PW = Σ [(MR_i + MR_{i-1})/2 * ΔP / (9.81 * 10)]
// MR = mixing ratio = 622 * e / (P - e)

Boundary Layer Depth

// Potential temperature
theta = T + (9.8 * h / 1000)

// Find height where theta > theta_sfc + 2°C

PART 3: LINK BUDGET ANALYZER (Point-to-Point Path Analysis)

Purpose

Complete point-to-point RF link analysis including path clearance, Fresnel zone obstruction, and margin calculations.

Path Geometry

Maidenhead Grid Decode: Field (20°×10°) → Square (2°×1°) → Subsquare (5'×2.5') → Extended

Distance: Great-circle haversine

Path Clearance Physics

Fresnel Zone Radius

function fresnelRadius(d1m, d2m, lambdaM) {
  return Math.sqrt(lambdaM * d1m * d2m / (d1m + d2m));
}
// λ = 0.3 / f_GHz

Earth Bulge Correction

function earthBulge(frac, distKm, K = 4/3) {
  const d1 = frac * distKm * 1000;
  const d2 = (1 - frac) * distKm * 1000;
  return (d1 * d2) / (2 * K * 6371000);
}

Effective K-factor from Surface Refractivity:

function effectiveKFactor(N_surface) {
  const N_std = 315;
  const dN_std = -40;
  const dN_est = dN_std - (N_surface - N_std) * 0.25;
  const Ke = 1 / (1 + 6371 * dN_est * 1e-6);
  return Math.max(0.5, Math.min(5.0, Ke));
}

function surfaceRefractivity(tC, rhPct, mslpHpa) {
  const T = tC + 273.15;
  const P = mslpHpa ?? 1013.25;
  const es = 6.112 * Math.exp(17.67 * tC / (tC + 243.5));
  const e = (rhPct / 100) * es;
  return (77.6 * P / T) + (3.73e5 * e / (T * T));
}

Knife-Edge Diffraction

function knifeEdgeLoss(v) {
  if (v <= -0.7787) return 0;      // Clear
  if (v <= 0) return -20 * Math.log10(0.5 - 0.62 * v);
  if (v <= 1) return -20 * Math.log10(0.5 * Math.exp(-0.95 * v));
  if (v <= 2.4) {
    const inner = Math.max(0, 0.1184 - (0.38 - 0.1 * v) ** 2);
    return -20 * Math.log10(0.4 - Math.sqrt(inner));
  }
  return 20 * Math.log10(v) + 13.0;  // Asymptotic
}

Diffraction Verdicts:

  • v < -0.7787: CLEAR (0 dB loss)
  • -0.7787 < v < 0: FRESNEL_MINOR (< 3 dB)
  • 0 < v < 2.4: FRESNEL_PARTIAL (3-10 dB)
  • v > 2.4: BLOCKED (>10 dB)

RF Link Budget

Free-Space Path Loss (ITU-R P.525)

function fspl(distKm, fGhz) {
  return 20 * Math.log10(distKm) + 20 * Math.log10(fGhz) + 92.45;
}

Gaseous Absorption (ITU-R P.676)

function gasLoss(distKm, rhoGm3, freq) {
  return (cfg.o2dBkm + cfg.h2oCoeff * rhoGm3) * distKm;
}
Frequency O₂ (dB/km) H₂O Coeff Notes
47 GHz 0.040 0.006 Clear window
24 GHz 0.006 0.0067 H₂O shoulder
10 GHz 0.003 0.0005 Clear window

EIRP

eirp = pDbm + gtxDbi - feedDb

Receiver Sensitivity

sensitivity = -174 + nfDb + 10 * Math.log10(bw)
// CW: bw=500 Hz, SSB: bw=2700 Hz

Duct Enhancement Estimate

function ductEnhancement(propScore) {
  if (propScore >= 80) return -14;  // 14 dB improvement
  if (propScore >= 65) return -10;
  if (propScore >= 50) return -6;
  if (propScore >= 33) return -2;
  return 0;
}

Calibrated against confirmed contacts.

Success Probability

function marginToSuccess(marginDb, propScore) {
  // Margin factor (piecewise linear)
  let marginPct;
  if (marginDb <= 0)       marginPct = 0;
  else if (marginDb <= 10) marginPct = (marginDb / 10) * 20;
  else if (marginDb <= 15) marginPct = 20 + ((marginDb - 10) / 5) * 20;
  else if (marginDb <= 20) marginPct = 40 + ((marginDb - 15) / 5) * 20;
  else if (marginDb <= 25) marginPct = 60 + ((marginDb - 20) / 5) * 20;
  else if (marginDb <= 30) marginPct = 80 + ((marginDb - 25) / 5) * 20;
  else marginPct = 100;

  // Propagation factor (±30% modulation)
  const propFactor = 0.70 + (propScore / 100) * 0.60;

  return Math.max(0, Math.min(99, Math.round(marginPct * propFactor)));
}

Margin → Success Calibration:

  • 0 dB → 0%, 10 dB → 20%, 15 dB → 40%, 20 dB → 60%, 25 dB → 80%, 30+ dB → 100%

Propagation Modulation:

  • Score 100 (excellent): ×1.30
  • Score 50 (neutral): ×1.00
  • Score 0 (poor): ×0.70

Integration: Data Flow

Weather Data (ASOS/NWS)
    ↓
[PropCast] → Composite Score 0-100
    ↓
    ├→ Real-time station dashboard
    └→ Input to LinkBudget (propScore parameter)

Sounding Data (IEM RAOB)
    ↓
[PropAnalyzer] → Refractivity profiles, ducts, inversions
    ↓
    ├→ Skew-T visualization
    ├→ dN/dh gradient analysis
    └→ Mechanism identification

Path Geometry + Elevation
    ↓
[LinkBudget] → Profile analysis, Fresnel zones, clearance
    ↓
    ├→ Diffraction loss calculation
    ├→ Link margin = RX Power - Sensitivity
    └→ Success % = marginToSuccess(margin, propScore)

Constants & Thresholds Reference

Constant Value Source
Humidity penalty 47 GHz 1.0 Minimal H₂O line sensitivity
Humidity penalty 24 GHz 1.6 Adjacent 22.235 GHz H₂O peak
Duct M-unit min depth 2 Noise filter
Inversion min strength 0.5°C Below is noise
Inversion height limit 5000m AGL Above irrelevant
Earth radius 6371 km WGS-84 mean
Standard K-factor 4/3 Standard atmosphere
Standard surface N 315 ITU-R P.453
Standard dN/dh -40 /km ITU-R P.453

ITU-R References

  • P.453-14: Radio refractivity
  • P.525: Free-space path loss
  • P.676: Gaseous absorption
  • P.526: Diffraction